This invention relates generally to lasers and more specifically to an apparatus for efficiently producing laser action from highly excited atomic ions throughout the visible, ultraviolet and vacuum ultraviolet regions of the light spectrum.
With the exception of the helium-neon gas laser, virtually all of the important visible and ultraviolet continuous-wave gas laser wavelengths belong to the spectra of ionized atoms. See, for example, U.S. Pat. No. 3,464,025. The design and construction of long lived reliable gas ion laser tubes has taxed the ingenuity of many inventors, scientists and engineers. The containment in a small capillary tube discharge of the extremely hot electrified gas needed to obtain laser action pushes the stability of known refractory laser tube materials to the limit. Two basic gas ion laser tube topologies became commercially available when fluid cooled fused silica capillary tube discharges were found to be very short lived. One type of gas ion laser tube described in U.S. Pat. No. 3,619,810 uses a stack of coaxially aligned graphite segments contained within a gas-tight glass tube. Another type of gas ion laser tube described in U.S. Pat. No. 3,760,213 employs a thick-walled high thermal conductivity ceramic capillary tube discharge in direct contact with the fluid coolant. The preferred embodiment uses beryllium oxide ceramic bore segments bonded together to provide the desired discharge length.
An improved type of gas ion laser described in U.S. Pat. Nos. 4,378,600 and 4,376,328 overcomes some of the limitations of the two prior art designs discussed above. The discharge tube taught in these two references is composed of refractory discs bonded to a ductile high thermal conductivity material, which in turn is bonded to a fluid cooled hermetically sealed insulating tube. A preferred embodiment uses carefully aligned refractory metal disc apertures brazed to copper discs which are in turn brazed at their outer circumferences to a gas-tight alumina tube with exterior fluid cooling.
Another improved type of gas ion laser described in U.S. Pat. No. 3,931,589 employs, as a source of electrons, a hollow cathode discharge centered coaxially about the optical axis.
The prior art gas ion lasers described in the references cited above are not very efficient, with only a small fraction (typically 0.01) of a percent of the electrical discharge energy appearing as laser output. A number of investigators have studied alternative excitation schemes in an attempt to improve the basic efficiency of this class of ion lasers, but to date the aperture-enhanced high current density plasma tubes have prevailed.
An analysis of the laser inversion mechanism for the singly ionized argon gas ion can be illustrated by referring to the generalized energy level diagram of FIG. 1. Alternative populating mechanisms are possible depending on discharge conditions and choice of ion species.
Examination of the energy levels shown in FIG. 1 indicates that electron energy levels well in excess of 20 electron volts are required to populate the upper laser states starting from the ground state of neutral argon. For current densities less than 100 amperes per square centimeter of beam cross section the peak of the Maxwellian-Boltzman energy distribution is approximately 3-5 electron volts, as shown in the shaded portion of FIG. 1A.
At higher current densities the electron temperature will increase as shown in FIG. 2, giving rise to a small increase in energetic electrons on the tail of the electron population distribution as shown in FIG. 3. It is clear that impractically large current densities would be required to appreciably increase the number of energetic electrons in a low pressure gas discharge at the energy of 20 electron volts. Thus, a practical upper limit is set on the laser output power and generation efficiency obtainable from capillary or apertured gas ion laser plasma tubes, this upper limit being typically 10.sup.-2 percent.
In summary, conventional gas ion lasers have a fundamental deficiency. Although the high current density provides ample charged particles, conventional gas ion lasers lack the energetic electrons required for efficient pumping of the upper laser states.